Not Applicable
Not Applicable
1. Field of the Invention
The present invention relates to an oil well casing centralizer coupling for use in cementing operations.
2. Description of Related Art
The process of drilling for oil is a multi-step process. First, a bore hole is drilled into the ground using a drill bit and drill pipe. Drilling mud is used for lubrication and pressure control during the drilling operation. Then, a casing is inserted into the bore hole. The casing is typically constructed from a plurality of threadable connected tubular casing sections. Often the insertion process requires significant rotation and axial reciprocation of the casing within the bore hole due to the friction generated between the walls of the formation and the pipe couplings or centralizer devices. The casing is centralized within the bore hole and is cemented into place by cement between the inner diameter of the bore hole and the outer diameter of the casing. The cement displaces the heavy, sticky drilling mud left over from the drilling operation. Preferably, the casing is rotated and/or reciprocated longitudinally to promote a uniform distribution of cement around the casing along its length. Once the casing is cemented in place, a smaller pipe (tubing) may be inserted into the casing for oil removal.
To center the casing within the bore, the industry has been to use centralizing blade assemblies that are attached to the outside of the casing. These blade assemblies are positioned along the length of the casing at regular intervals and held in place with setscrews, clamping collars, and the like. One problem with these blade assemblies is that they commonly became loose due to the casing rotation and reciprocation. When the blade assemblies become loose, they often slide along the casing. When this happens, they tend to bunch up at one or more locations. This results in poor centralizing of the casingxe2x80x94and consequently, a poor cementing job. Another problem with these blade assemblies is that the setscrews often cut the casing as the blade assemblies slide under high torque rotation of the casing. These cuts reduce the integrity of the casing. Another problem with the prior art is that these devices do not address the need to promote smooth, non-turbulent, reduced-friction flow around the centralizer, allowing the sticky drilling mud to release, which reduces the pumping pressure.
Finally, the setscrew bodies tend to disrupt and unbalance the flow path of the cement around the casing sections, which also produces a poor cementing job.
An exemplary centralizer is the Lirette et al. centralizer of U.S. Pat. No. 5,575,333. This centralizer uses movable, flexible spring bows to space the casing from the sides of the bore hole. The Lirette et al. centralizer includes threaded ends that allow it to function as a casing coupling that connects sections of an inner casing within an outer casing. However, such movable, flexible spring bows tend to prohibit the rotation of a casing within a bore hole because the spring bows are not typically able to withstand the high torque developed by rotation of the casing relative to the inner surface of the bore hole.
Other forms of centralizers use rigid blade assemblies, which have been used on drilling tool couplings with female threads on a first end and male threads on a second end. Examples of this design are found in the following U.S. Pat. No. 4,595,058 to Nations teaches a turbulence cementing sub. This device is placed in the casing string to act as a centralizer. The device has a number of ribs formed on the outside of the device. The ribs are laid out so that some are angled in one direction and others are angled in the opposite direction. This is claimed to create a turbulent effect in the cement that supposedly eliminates having to turn or reciprocate the pipe. However, the effect is to reduce the smooth flow of the cement, raise the pumping pressure and impede the displacement of the drilling mud.
U.S. Pat. No. 2,309,791 to Sanders is a coupling type device that is screwed into a pipe string. It has ribs to hold the pipe away from the walls of the well. U.S. Pat. No. 3,762,472 to Alexander Jr., is a cylindrical device that has a larger diameter than the pipe, but a smaller diameter than a coupling. This device is designed to slide on the pipe between couplings. The device has ribs attached to help center the pipe. The problem with this device is that the force on the device maybe such that it is slammed into the couplings, which can damage them. In addition, the device cannot be used in a flush line system because, in such a system, there are no couplings used. U.S. Pat. No. 5,697,442 to Baldridge teaches a float shoe or collar that has ribs to act as a centralizer. The Baldridge patent, however, covers a device for washing the well bore walls. Finally, U.S. Pat. No. 4,995,456 to Cornette et al. teaches a gravel packing system that uses spaces with helical ribs on them to help spread the gravel slurry around the casing. Although this operation appears to be similar to the cementing operation, it is quite different and exactly the opposite effect from that desired.
All these devices have problems. Mostly, these problems arise because these centralizers are external add-ons to the casing. As such, they protrude into the well space, they do not hold their position in the well, and they dig into the casing surface when they move, and they do not allow cement to be applied uniformly to the casing. Moreover, their roughness and uncoated surfaces increase cement pumping pressure and lower flows leaving some of the drilling mud in place, which can lead to failed cementing and a non-usable well.
The devices that use some type of coupling with ribs are more effective, but tend to be large, excessively long and difficult to handle. Consequently, these ribs can increase flow restrictions. Moreover, all of these devices need some type of smooth cylindrical surface to accommodate a wrench when the device is placed into or removed from the string.
The centralizer coupling of the present invention overcomes all these problems. It is able to withstand the high torque rotation and reciprocation of the casing without slippage, while allowing cement to flow smoothly, evacuating the drilling mud easily, reducing insertion and pumping pressures, and increasing cement flow rates.
The oil well casing centralizer of the present invention is a threaded coupling that has a pair of threaded ends. A number of rigid centralizer ribs are attached or formed on the coupler. Because the coupler has a small length, a special wrench-adapter is used to attach the coupler to the casing string. This adapter fits around the coupler and allows an ordinary wrench to hold the coupler while the casing string is being made up.
The coupling also has a gas seal to contain natural gas within the casing without allowing leakage and a torque limiter to ensure that the coupler does not destroy the casing when the casing is rotated in the earth.